IT'S not every day that fans in 75 countries gather at the same time to watch their favourite show.

But Doctor Who is one series able to command such a global audience, and this week the British sci-fi franchise celebrates 50 years on the small screen.

Over the years viewers have watched 11 actors portray the time-travelling alien's adventures in space and time, battling various forces of evil, the most famous of which are the Daleks.

"Part of its success is there's a Time Lord whizzing around solving problems with a pocket full of odds and ends and a screwdriver," Terry Molloy, who played the Daleks' evil creator Davros from 1984 to 1988, told The Guide.

"His technology is his brain that he employs to save people."

Molloy is celebrating 30 years since he joined the series and he recently met Australian fans at Supanova in Brisbane before flying back to the UK for the BBC's three-day Doctor Who 50th anniversary convention.

"Doctor Who is a giant extended family. All of us have stories to tell," he said.

"What a lot of the conventions are about is the sharing of an experience you've had, either being in it or being a fan of it. The fans kept it going in the dead years (when the BBC axed the show in 1989). It was kept alive in the hearts and minds of fans who were devastated when it was taken off the screen."

The current series, launched in 2005, has breathed new life into the franchise.

Out-going Doctor Matt Smith and previous Doctor David Tennant star in the 50th anniversary special airing around the world on Sunday, including in Australia on ABC1.

All of reality is at stake in the anniversary episode, which also stars John Hurt, Jenna Coleman and Billie Piper.

Molloy can't wait for the new era Peter Capaldi will usher in as the 12th Doctor.

"He's a brilliant actor, an Oscar winner and from the age of 15 he was a fan boy," he said.

"He's been preparing for the Doctor all this life. It will be good for kids to have a slightly older Doctor.

"Capaldi is the same age as (William) Hartnell and (Patrick) Troughton were (when they played the first and second Doctors respectively). I think this will be an era where the Doctor has a degree of gravitas because of his age as well as a being the Doctor and a Time Lord."

Doctor Who: The Day of the Doctor - ABC1 - Sunday at 5.50am Qld, 6.50am NSW and repeated at 7.30pm